A light-emitting device (LED), as used herein, is a semiconductor light source including a light emitting diode and optionally photoluminescence material, also referred to herein as phosphor, for generating a light at a specified wavelength or a range of wavelengths. LEDs are traditionally used for indicator lamps, and are increasingly used for displays. An LED emits light when a voltage is applied across a p-n junction formed by oppositely doping semiconductor compound layers. Different wavelengths of light can be generated using different materials by varying the bandgaps of the semiconductor layers and by fabricating an active layer within the p-n junction. The applied voltage controls the intensity of the light. Additionally, the optional phosphor material changes the properties of light generated by the LED.
In LED displays, multiple LEDs are often used in to form a color image pixel. In one example, three separate light sources for red, green, and blue in separate LEDs having different compositions, individual optics and control are grouped or driven together to form one pixel. The pixel can generate a full spectrum of colors when individual LEDs are activated and controlled. As this display ages, the white point of the display can move as the different color LEDs age at different rates. Also the display resolution is low because one pixel includes three or more separate LEDs and their associated optics and circuitry.
An LED can also be used to generate white light. A white light LED usually generates a polychromatic light through the application of several phosphors, either in a mixture or several phosphor layers. The phosphors Stokes shift blue light or other shorter wavelength light to a longer wavelength. The perception of white may be evoked by generating mixtures of wavelengths that stimulate all three types of color sensitive cone cells (red, green, and blue) in the human eye in nearly equal amounts and with high brightness compared to the surroundings in a process called additive mixing. The white light LED may be used as lighting, such as back lighting for various display devices, commonly in conjunction with a liquid crystal display (LCD). There are several challenges with LED backlights. Good uniformity is hard to achieve in manufacturing and as the LEDs age, with each LED possibly aging at a different rate. Thus it is common to see color temperature or brightness changes in one area of the screen as the display age with color temperature changes of several hundreds of Kelvins being recorded.
Higher display resolution and consistent aging are increasingly required of LED displays. An LED design that allows high resolution color displays without an LCD and/or allows adjustment to ensure uniform color temperature after manufacturing and as the white LED ages is sought.